The present invention relates, in general, to optical information storage and processing, and more particularly to the use of oriented organic and biological molecules as media for optical information storage and optical information processing.
With the rapid growth of photonics technology, considerable effort is being directed toward the development of new materials for optical information storage and processing, and interest has focused on both organic and biological molecules as possible media. The search has already encompassed several materials, including bacterial membranes, which have been used for a variety of purposes, including second harmonic generation and dynamic holography. Of particular interest has been the protein known as bacteriorhodopsin (BR), which exhibits important photochromic and optoelectrical properties. This material has extremely large optical nonlinearities, and is capable of producing photoinduced electrical signals whose polarity depends on the prior exposure of the material to light of various wavelengths as well as on the wavelength of the light used to induce the signal, and these properties are useful for information storage and computation. Numerous applications of this material have been proposed, including its use as an ultrafast photosignal detector, its use for dynamic holographic recording, and its use for data storage.
Although the present invention is not limited to a particular material, it will be described in terms of its application to bacteriorhodopsin, which is the preferred material since the rhodopsins are a biologically important class of proteins which includes, for example, the visual rhodopsins which are responsible for the conversion of light into nerve impulses in the image resolving eyes of mollusks, anthropods, and vertebrates. Bacterial rhodopsins represent a related class of proteins that serve both photosynthetic and phototactic functions. The best known of this latter group is bacteriorhodopsin, which is very unique, since it is the only protein found in nature in a crystalline membrane. This crystalline membrane, which is called the "purple membrane", has BR as its sole protein component, and converts light into energy via photon-activated transmembrane proton pumping. Upon the absorption of light, the BR molecule undergoes several structural transformations in a well-defined photocycle wherein energy is stored in a proton gradient formed upon absorption of light energy. This proton gradient is subsequently utilized in the BR cell to synthesize energy-rich ATP.
The structural changes which occur in the process of light-induced proton pumping of BR are reflected in alterations of the absorption spectra of the molecule. These changes are cyclic, and under usual physiological conditions bring the molecule back to its initial BR state after the absorption of light in about 10 milliseconds. Although the molecular details of this photocycle are not fully known, most of the intermediate states which occur are well established. Thus, in less than a picosecond after BR absorbs a photon, the BR produces an intermediate, known as the "J" state, which has a red-shifted absorption maximum. This is the only light-driven event in the photocycle; the rest of the steps are thermally driven processes which occur naturally. The first form, or state, following the photon-induced step is called "K", which represents the first form of light-activated BR that can be stabilized by reducing the temperature to 90.degree. K. This form occurs about 3 picoseconds after the J intermediate at room temperature. Two microseconds later there occurs an "L" intermediate state which is, in turn, followed in 50 microseconds by an "M" intermediate state. The steps in the photocycle not only are sensitive to temperature, but can be drastically altered by pH, humidity, and the resuspension of the membranes in D.sub.2 O. The intermediates L and M are particularly susceptible to these chemical environmental changes in the membrane.
There are two important properties associated with all of the intermediate states of this material. The first is their ability to be photochemically converted back to the basic BR state. Under conditions where a particular intermediate is made stable, illumination with light at a wavelength corresponding to the absorption of the intermediate state in question results in regeneration of the BR state. addition, the BR state and intermediates exhibit large two-photon absorption processes which can be used to induce interconversions among different states.
The second important property is light-induced vectorial charge transport within the molecule. In an oriented BR film, such a charge transport can be detected as an electric signal. The electrical polarity of the signal depends on the physical orientation of molecules within the material as well as on the photochemical reaction induced. The latter effect is due to the dependence of charge transport direction on which intermediates (including the BR state) are involved in the photochemical reaction in question. For example, the polarity of an electrical signal associated with the BR.fwdarw.M photochemical reaction is opposite to that associated with the M.fwdarw.BR photochemical reaction. The latter reaction can be induced by light with a wavelength around 412 nm and is completed in 200 ns.
For the information storage and processing of the present invention, the intermediate states of bacteriorhodopsin are of particular interest, for at 77.degree. K., the BR state and the K intermediates can be switched back and forth by the use of light with wavelengths Corresponding to the absorption maxima of these intermediates. The switching time is a few picoseconds, so that this switching is very attractive for fast optical information processing. However, it does require very low temperatures to stabilize the K intermediate, and its absorption spectrum has a large overlap with that of the BR state, which reduces the contrast ratio; that is, the overlap prevents complete switching of all molecules between these two states. Because of these complications, switching between the BR and the M intermediate is more desirable.
The quantum yields for the forward and back photoreaction in switching between BR and M are similar to those for switching between BR and K. Although its switching time is not as fast as K, the absorption band of M is distinct from that of BR and this allows complete switching of the BR molecules to the M intermediate and reduces, in comparison to other photochromic materials, the power needed to accomplish this optical switching. Furthermore, the time required to initiate the transition between BR and M is much shorter than the total transition time because of the fact that once BR is photochemically switched to the primary photochemical product J, which takes place in less than 1 picosecond, all molecules that have reached the J state will then automatically thermally relax to the M state. An additional advantage of the M state is that it can be stabilized at a much higher temperature, about 208.degree. K., than is required to stabilize the K state.
In addition to the large quantum yields and distinct absorptions of BR and M, the BR molecule has several intrinsic properties which are of-importance in optics and which, therefore, make this material preferred for the present invention. First, this molecule exhibits a large two-photon absorption cross section. Second, the crystalline nature and adaptation to high salt environments makes the purple membrane very resistant to degeneration by environmental perturbations and thus, unlike other biological materials, it does not require special storage. In fact, dry films of purple membrane have been stored for several years without degradation. Furthermore, the molecule is very resistant to photochemical degradation, and in experiments a film of purple membrane was switched between its BR and M states more than 10.sup.6 times with no noticeable change. The use of BR is also important to the invention because both the spectrum and kinetic aspects of the BR photocycle can readily be modified by replacing the light-absorbing component of the protein. This component is a retinal (vitamin A)--like chromophore, which can be replaced by natural and synthetic analogs which can shift the BR spectrum to virtually any color. Also, genetic mutants of BR can be produced by biotechnological procedures to affect both the kinetic and spectral properties of BR. Finally, BR and its associated M state have a very large second order nonlinear susceptibility which can be used to read without erasing.